Emission control systems of motor vehicles, such as motor vehicles propelled by gasoline-fueled or diesel-fueled internal combustion engines, may include one or more catalysts configured to reduce the level of emissions produced during fuel combustion. For example, a SCR catalyst may be included in an emission control system to reduce levels of nitrogen oxides (NOx) emitted by the engine following fuel combustion. A reductant injected into the exhaust passage upstream of the SCR catalyst is adsorbed onto a substrate within the SCR catalyst, and as exhaust gas flows through the SCR catalyst, the reductant stored in the SCR catalyst reacts with NOx in the exhaust gas. The reductant may be urea, which transforms into ammonia (NH3) prior to being adsorbed onto the surfaces within the SCR catalyst. The NH3 stored by the SCR catalyst reacts with NOx in the exhaust gas to create byproducts such as N2 and H2O.
The amount of reductant injected upstream of the SCR catalyst as well as the reductant storage capacity of the SCR catalyst impacts the NOx conversion efficiency of the SCR catalyst. The storage capacity of an SCR catalyst varies based on the temperature of the SCR catalyst. For example, if a large amount of reductant is injected upstream of the SCR catalyst and stored in the SCR catalyst, a high NOx conversion efficiency may be achieved. However, if a large amount of reductant is injected upstream of the SCR catalyst and the SCR catalyst does not have sufficient storage capacity, some of the reductant may “slip” out of the SCR catalyst and then out of the exhaust tailpipe, resulting in undesirable exhaust emissions (e.g., excessive NH3 emissions). Conversely, if a small amount of reductant is injected, NOx conversion efficiency may decrease, as there may not be enough reductant stored in the SCR catalyst to react with and reduce the NOx in the exhaust gas flowing through the SCR catalyst. This may result in harmful NOx emissions from the exhaust tailpipe of the vehicle.
In some examples, the vehicle controller is programmed to maximize NOx conversion efficiency and minimize reductant slip by adjusting the reductant injection mass flow rate based on the relationship between a modeled reductant storage level and a reductant storage setpoint. For example, as it may be difficult to directly measure the level of reductant stored in the SCR catalyst during vehicle operation, a control-oriented model may be used to estimate the level of reductant stored in the SCR catalyst at a given time. Further, as NOx conversion efficiency and reductant slip are highly dependent on the temperature of the SCR catalyst, the temperature must also factor into the control strategy. Some control-oriented models utilize a zero-dimensional lumped parameter structure for the sake of simplicity and computational efficiency. In such models, a single node defines all dynamics of the SCR catalyst, and axial distribution of reductant storage and axial/radial temperature variations are ignored. Thus, such models may be restricted in their ability to capture the effects of temperature gradient and/or reductant storage distribution on catalyst dynamics.
Other example control-oriented models include distributed models, which discretize the SCR system into a number of elements or slices each having respective inputs, outputs, and internal states such as reductant storage level and substrate temperature. For example, U.S. 2014/0032189 describes a method for a model-based determination of temperature distribution within an exhaust gas post-treatment unit, where the model virtually segments the unit axially and radially. During steady operating states, the radial temperature distribution from the unit to its surroundings is taken into account, whereas during non-steady operating states, the heat transfer from the exhaust gas flowing axially through the unit to the unit is taken into account.
However, the inventors herein have recognized potential issues with the control-oriented models described above. As one example, models focusing on temperature distribution alone may not adequately account for reductant storage distribution in an SCR catalyst. As another example, the radial elements modeled in the above approaches have fixed dimensions, and thus cannot accurately approximate the changes in radial dynamics of an SCR catalyst that occur in real time. Only the inventors herein have recognized that these and other issues with prior approaches may be addressed by a method for a vehicle engine emission control system in which various vehicle operating parameters are adjusted to maximize the performance of an SCR catalyst, where the adjustments are based on a comparison of an estimated spatial distribution of reductant stored in an SCR catalyst with a desired spatial distribution of reductant stored in the catalyst. For example, if radial adjustment of the spatial distribution is desired, a pressure at which reductant is injected into the SCR catalyst is adjusted. Further, if axial adjustment of the spatial distribution is desired, exhaust gas temperature and/or NOx concentration is adjusted. For example, non-uniform radial and axial reductant storage and temperature profiles may model actual catalyst dynamics, such that SCR control more readily accommodates real-time catalyst and vehicle conditions. The resulting robust SCR control may advantageously increase NOx conversion efficiency and decrease ammonia slip so as to meet increasingly stringent emission control regulations.
Only Applicant has identified strategies for adjusting vehicle operating parameters based on radial and axial models of the SCR catalyst in order to achieve a desired performance of the SCR catalyst. For example, Applicant has recognized that the radial distribution of reductant at front face of an SCR catalyst may be actively adjusted via adjustment of the dosing pressure of reductant injected upstream of the SCR catalyst, where the adjustment is optionally timed to correspond with a desired exhaust flow rate. However, Applicant has recognized that adjustment of reductant injection alone may not provide sufficient control to achieve desired reductant storage setpoints throughout the SCR catalyst, for example because adjustment of reductant injection alone cannot influence the axial storage location of reductant. To overcome this limitation, Applicant has recognized that active perturbation of the temperature of exhaust gas entering the SCR catalyst may cause reductant stored in a front portion of the SCR catalyst (e.g., closer to an inlet of the catalyst with respect to the flow of exhaust gas through the catalyst) to be moved towards a back portion of the SCR catalyst (e.g., closer to the outlet of the catalyst), thereby advantageously increasing NOx reduction efficiency, especially at high rates of exhaust gas flow through the catalyst. Further, Applicant has recognized that active perturbation of the NOx concentration in the exhaust gas entering the SCR catalyst may advantageously reduce back-skewing of stored reductant (e.g., the tendency of reductant to be stored toward the back of the catalyst), thereby advantageously decreasing reductant slip and increasing fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.